1. Field of the Invention
The invention relates to a device for treating a web surface in a plasma enhanced process, it serves in particular for coating one surface of a web of a flexible material in a plasma enhanced chemical vapour deposition process, e.g. coating a web of polymer film or sheet material with silicon oxide in order to improve its barrier properties.
2. Description of Related Art
A device of the named kind is e.g. described in the publication EP-0605534 (or U.S. Pat. No. 5,224,441, BOC). The device comprises a powered electrode in the form of a rotating drum and a grounded shield shaped like about half a hollow cylinder and arranged to form together with the drum a space having a width of between 1 and 30 cm. On the side of the shield facing away from the drum a plurality of permanent magnets is arranged such that alternating magnetic poles face towards the drum. For supplying a process gas or process gas mixture to the gap between the drum and the shield, the device comprises a gas supply line extending parallel to the drum axis in a central part of the shield. The drum, the shield with the magnetic means and the gas supply line are arranged within a vacuum chamber comprising means for removing the supplied process gas from the chamber in order to maintain a constant reduced pressure within the chamber.
The web of flexible material is conveyed through the space between the drum and the shield being supported and transported by the drum, i.e. lying on the circumferential surface of the drum, while a plasma is maintained in this space by powering the drum with a high frequency alternating voltage and by supplying a process gas mixture to the plasma. The plasma enhanced process described in the named publication is a plasma enhanced chemical vapour deposition of silicon oxide on a polymer film using a process gas mixture comprising an organosilicon compound, oxygen and an inert gas.
Another device for treating a web of flexible material in a plasma enhanced process is described in the publication U.S. Pat. No. 4,968,918 (Kondo et al.). The device comprises a plurality of stationary antenna electrodes extending radially from a central power introducing member and being arranged in a substantially cylindrical vacuum chamber. A grounded counter electrode is arranged between each two neighbouring antenna electrodes. Gas feed lines extend axially e.g. along the outer rim of the grounded electrodes. The web of flexible material is guided in a sort of zig-zag-path between the antenna electrodes and grounded electrodes either supported by the antenna electrodes or by the grounded electrodes. The process being carried out with the described device is an etching process using oxygen as a process gas, a plasma polymerisation, a chemical vapour deposition or the like.